Objectives (1) To characterize and quantify species differences in the reproductive biology and reproductive performance of rhesus macaques and stumptailed macaques; (2) to assess species differences in the susceptibility of major reproductive events to environmental influences including temperature, quality of light, and photo periodicity. ABSTRACT:The utility of primates as animal models in reproductive research depends upon a thorough understanding of parameters of the reproductive biology of the species studied, and adequate characterization of similarities and differences with other primate species and in relation to humans. Important differences exist, for example, between two Cercopithecid species that have been heavily utilized in research on reproduction the rhesus macaque, Macaca mulatta, and the stumptailed macaque, Macaca arctoides these include differences in the extent to which major reproductive events are susceptible to environmental influence. Of particular interest are marked species differences in seasonality of birth patterns and in extent of reproductive synchrony observed. Because aspects of reproduction in mammals are well-recognized as being subject to the influence of nutrition, temperature, light, energetic requirements, space, population density, and social environment, it is difficult to make valid comparisons of the reproductive patterns of different populations without generous assumptions. The large groups of rhesus and stumptailed macaques at the WRPRC's Vilas Zoo facility provide a rare and valuable opportunity to understand inherent species differences in reproduction from the perspective of a long-term data base on populations in the same climate, with identical physical environment and diet, and with similar group structures. Elucidation of inherent species differences in fundamental reproductive parameters, together with consideration of data from field studies, will provide further insights into the evolutionary and ecological influences shaping the reproductive "strategy" of each species. For example, stumptails may be shown to share with humans relatively delayed reproduction, lack of reproductive synchrony, similar dissociation between birth seasonality and environment and between mating activity and environment, longer inter-birth interval and relatively extended care of infants, with fewer infants per lifetime and greater investment in each. The study populations are particularly valuable in the inclusion of many older females data on age-related trends in productivity and patterns of reproductive senescence, overlapping a parallel study of female reproductive success, will provide further insights into species differences in reproduction within an evolutionary framework. Key Words reproduction, paternity testing, minisatellite DNA, genetic transmission, genetic diversity